Perishable products are often transported from one point to another in refrigerated trucks, trailers or the like. For certain perishables, it is desirable to maintain them in a cold but unfrozen state, substantially around 32.degree. F. Factors such as the temperature of outside air, the starting temperature of the perishable, and the capacity of the refrigeration system effect the ability and speed of the transportation unit in maintaining the perishable at the desired set point temperature. Accordingly, it is frequently desirable to reach the setpoint temperature, which is typically at or around 32.degree. F., by pulling down the temperature in the trailer below the set point. However, an undesirable result of such a process is the top freezing of perishable goods, as the supply duct is typically positioned toward the ceiling of the refrigerated trailer, over the goods. As a result, the top portions of the perishable products may be frozen while the underlying products are maintained at the desired setpoint temperature.
A known method for substantially preventing top-freezing includes the use of a control system which senses supply air temperature to determine if the same is detected to have fallen below a pre-selected floor temperature which represents the safety threshold for preventing product damage. Based on a methodology including consideration of the supply air temperature, return air temperature, and the setpoint temperature, and other determinable factors, such as time for top freezing to occur and desired rate of pull-down, the temperature differential between the supply air and the setpoint temperature can be adjusted to achieve the desired results. However, one negative result of such temperature differential adjustments is a reduced capacity of the refrigeration system in cooling the perishable products, leading to the necessity for once again substantially decreasing the supply air, providing a highly fluctuating system temperature profile. While this system is effective in preventing top-freezing, the lack of a more direct capacity control function prevents achievement of a more stable temperature profile.
There exists a need, therefore, for a control system for a transport refrigeration unit, having an additional capacity control function, which prevents top-freezing and allows for stable system temperature control.